Municipal waste is globally a growing problem. Landfills generate methane that is a green house gas concern. In addition, the tipping fees for landfills are increasing due to land constraints or governmentally mandated closures, which are in place in multiple countries, particularly in Asia and in the European Union, or will be in the near future. Incineration can be used, but this is not a good option for the future due to environmental concerns.
Gasification is a process that converts carbonaceous materials, such as coal, petroleum, or biomass, into gases, such as carbon monoxide and hydrogen, by reacting the raw material at high temperatures with a controlled amount of oxygen. The resulting gas mixture is called synthesis gas or “syngas” and is itself a fuel. Gasification is a very efficient method for extracting energy from many different types of organic materials, and also has applications as a clean waste disposal technique.
Generally, the gasification process consists of feeding carbon-containing materials into a heated chamber (the gasifier) along with a controlled and limited amount of oxygen and steam. At the high operating temperatures created by conditions in the gasifier, chemical bonds are broken by thermal energy and by partial oxidation, and inorganic mineral matter is fused or vitrified to form a molten glass-like substance called slag or vitreous frit. With insufficient oxygen, oxidation is limited and the thermodynamics and chemical equilibrium of the system shift reactions and vapor species to a reduced, rather than an oxidized state. Consequently, the elements commonly found in fuels and other organic materials end up in the syngas.
The advantage of gasification is that using the syngas in a gas turbine or reciprocating engine can give a higher overall thermal efficiency (combined cycle) than direct combustion in a boiler configuration producing thermal energy with conversion in a steam turbine (simple cycle). As mentioned, syngas may be burned directly in an internal combustion engines or gas turbines, or converted via the Fischer-Tropsch process into synthetic fuel. Gasification can also begin with materials that are not otherwise useful fuels, such as biomass or organic waste. In addition, it is possible to separate out corrosive or harmful emissions post gasification but prior to combustion. This produces a very clean syngas resembling natural gas. In the traditional direct combustion methods, such as incineration, the flue gases have to be cleaned post combustion often resulting in poor emission results and high cost.
Gasification of fossil fuels is currently widely used on industrial scales to generate electricity. However, almost any type of organic material can be used as the raw material for gasification, such as wood, biomass, or even plastic waste. Thus, gasification may be an important technology for renewable energy. In particular biomass gasification is carbon neutral.
Waste gasification technology has been developing for the past 20+ years. In fact, there are about 80 plants now operating or under construction in Japan, which will utilize some form of gasification technology. Most gasifiers operate at near atmospheric pressure and require some energy in addition to the energy contained in the waste stream. Many gasifiers use coke, coal, natural gas or plasma arc torch to provide the additional energy.
Utilizing a plasma arc torch to gasify a material is a technology that has been used commercially for many years. In all current designs, the plasma arc torch is placed towards the bottom of the plasma arc reactor with the plasma plume focusing on the slag or bed. Most plasma arc reactors produce a high quality syngas that can be used as a building block for other chemical manufacturing processes or as a fuel for energy production. Many feeds containing hydrocarbons, such as oil, coal, refinery residuals, and sewage sludge have all been successfully used in gasification operations. It is sometimes desirable to convert a hazardous stream of material into a useable product by gasifying the material. Upon gasification, the hazardous material, or feed, will typically be converted into a useable syngas and a useful molten material, or a molten glass-like substance called slag or vitreous frit. Because the slag is in a fused, vitrified state, it is usually found to be non-hazardous and may be disposed of in a landfill as a non-hazardous material, or sold as an ore, road-bed, or other construction material. It is becoming less desirable to dispose of waste material by incineration because of relatively low combustion efficiency, the heavy pollution and the further waste of disposing, as a residual waste, material that can be converted into a useful syngas and solid material.
The gasification system can be designed for a variety of feedstock, including waste. Once the gasifier is designed, however, varying fuel composition may present operational problems because of difficulty maintaining proper oxygen balance as the fuel composition changes. This is especially true for untreated municipal waste, where the composition can drastically change depending on the location from where waste is collected.
Therefore, a new technology is needed that can provide a cost effective and environmentally sound process to destroy municipal and industrial waste. This invention describes the design of a new gasifier and gasification system that can help solve this global problem.